In a fault condition with welded contacts, the electrical contactor provides “free” un-metered electricity to a premises. A dangerous shock hazard can also occur if the Load, that is thought to be disconnected safely, is still live at mains voltage.
Many known electrical contactors are capable of satisfactorily switching nominal current at around 100 Amps or 200 Amps for a large number of Load-switching cycles. The switching is undertaken by special silver-alloy contacts containing certain additives, which prevents welding. The switching blades or arms are configured to be easily actuated for the switching function, with minimal self-heating losses at the nominal currents concerned.
Most electricity meter specifications not only stipulate satisfactory nominal-current endurance switching lifetimes without the contacts welding, but also demand that at moderate short-circuit fault conditions they must also not weld, and must open on the next actuator-driven pulse. At much higher related “dead-short” conditions the switch contacts may weld, but must remain intact. In other words, not explode or emit any dangerous molten material during the “dead-short” duration, until protective fuses rupture or circuit breakers drop-out and safely disconnect the supply to the Load. This short-circuit withstand duration must be for a maximum of six cycles of the AC mains supply.
In North American electricity metering, domestic 2-phase supplies are fed via a three-wire cable from a heavy-duty street-side utility transformer to the metered premises at 115 V AC per phase, being 180 degrees apart, with-respect-to a central Neutral/Earth connection. For moderate loads at 115 V AC, each metered phase is fed via ring-main wiring to distributed sockets in the premises. However, all power-hungry loads such as washing machines, clothes driers, space heaters, pool heaters and air-conditioners, for example, are connected across both phases at 230 V AC, with a maximum Load capability of 200 Amps. Therefore, a robust 200 Amps two-pole contactor is required within the meter for performing the Load-disconnect function, as and when demanded.
In Europe and a majority of other territories worldwide, the dominant supply is single-phase 220 V AC at 100 Amps, and more recently 120 Amps, in compliance with the IEC 62055-31 specification. In North America and a few other countries using an equivalent system, the supply is two-phase 230 V AC at 200 Amps. This latter case is governed mainly by the ANSI C12.1 metering specification. Safety aspects are covered by other related specifications, such as UL 508, ANSI C37.90.1, IEC 68-2-6, IEC 68-2-27, IEC 801.3.
It is known from British patent 2413703 to BLP Components Limited of Newmarket, United Kingdom, to provide a bi-blade arrangement of parallel movable spring copper blades having movable contacts opposing a corresponding fixed contact. Opposing pairs of the spring copper blades are aligned with each other across the fixed contacts. In a basic 100 Amp switch, two spring copper blades and two fixed contacts are utilised, resulting in a total of four contacts with 50 Amps flowing in each parallel blade.
In a second higher nominal-current embodiment, constituting a 200 Amp switch, each spring copper blade is sub-divided into two sprung sub-blades having a movable contact at each end. Each sub-blade is provided as part of a pair aligned and opposing each other across a fixed terminal member carrying associated fixed contacts. Each switch therefore has eight contacts, and a 2-pole 2-phase Load-disconnect contactor therefore comprises sixteen contacts in total.
Such current sharing between blades significantly reduces contact repulsion forces for more reliable switching, minimal self-heating, and non-welding at the higher Nominal and short-circuit currents.
A problem associated with the higher current 200 Amp 2-pole meter Load-disconnect contactor is the number of blades and contacts required. The increased number of blades necessitates a higher quantity of electrically conductive metal, in this case copper, and the increased number of contacts requires a greater silver content. This increases manufacturing costs substantially.
The known 100 Amp switch design from GB2413703 using simple parallel spring copper bi-blades is limited by the geometries and gap between each facing blade in the bi-blade set. Each bi-blade pair is capable of generating a certain magnetic attraction force at high shared current, one with-respect-to the other, balanced and acting against the contact repulsion forces. This ensures that the contacts remain closed during short-circuit faults. It is extremely difficult to configure the bi-blade pair to correctly balance the ratio of forces for a particular configuration, and given the limited space within the contactor casing. For the high current 200 Amp switch design, it was therefore convenient to utilise opposing aligned sub-blade pairs to achieve the desired switching characteristics.